The present invention relates to means and methods of attenuating electrical signals over a predetermined frequency range. In a typical application, the invention relates to signal attenuation in communication systems having forward and return paths, such as those commonly used in CATV applications, and more particularly to novel circuits and methods for introducing a desired step attenuation (flat loss) in the return path signal or in both forward and return path signals.
In multi-channel communication systems it is common to transmit signals at frequencies higher than a predetermined value from an input to an output end over what is termed a forward path, and to transmit signals at frequencies below the predetermined value from the output back to the input end over a so-called return path. In order to improve network performance and reduce noise ingress, it is desirable to attenuate signals in the return path by a substantially uniform amount over the range of return path frequencies. Forward path signals are most commonly transmitted substantially unimpeded, i.e., without attenuation, although in some applications it may be desirable to attenuate the signals over the full frequency band of both forward and return path signals. Such communication systems, particularly as employed in the field of CATV, are well known and a number of systems have been devised to provide the step attenuation (flat loss) in the transmitted signals.
Diplex filters, for example, have been used for processing both the forward and return paths in CATV communication systems. U.S. Pat. No. 5,745,838 to Tresness et al. discloses a special form of diplex filter including fixed-value resistors in a TEE or PI configuration which attenuates signals in the return path by a predetermined amount. The fixed resistors providing the attenuation operate, of course, independently of frequency.
The present invention is directed to apparatus and methods for effecting step attenuation of return path, or both forward and return path signals in two way communication systems through the use of a return path filter network having only frequency dependent components. The invention provides a number of advantages over prior art circuits which perform this function using a fixed resistors. Such advantages include: reducing the number of circuit components, reducing filter size and cost, and/or increasing the filter""s electric surge and power handling capabilities.
The filter apparatus of the invention includes input and output ends connected, for example, to the head end and subscriber terminal, respectively, of a cable television system, as well as the usual forward and return communication paths. The filter""s forward path includes series-connected capacitors, as in the prior art, providing a highpass filter for passing signals above a predetermined frequency, e.g., RF television signals, to the subscriber end. The invention is based on designing the return path filter network using frequency dependent components to provide a desired amount of signal attenuation. In the disclosed embodiment, electromagnetic RF chokes replace traditional, fixed-value resistors and associated inductors, thus providing the desired attenuation while realizing the aforementioned improvements over corresponding, prior art filters. The RF chokes provide a combined response having both inductance and impedance characteristics, which is frequency dependent, as opposed to the purely resistive, frequency independent response of the prior art return path filters. By choosing from a wide variety of commercially available RF chokes, the amount of flat loss as well as optimization of overall network performance can be achieved.